Trigger:
-----------------------
trigger AccountTrigger on Account (after insert, before update) {
    // On Before Update
    if(Trigger.isBefore && Trigger.isUpdate) {
        AccountTriggerHandler.onBeforeUpdate(Trigger.New);
    }
    
    // On After Insert
    if(Trigger.isAfter && Trigger.isInsert) {
        AccountTriggerHandler.onAfterSave(Trigger.New);
    }
    
}


Handler:
-------------------
public class AccountTriggerHandler {
    
    // Method to execute on After Insert
    public static void onAfterSave(List<Account> accounts) {
        // Map to update the List of Accounts
        Map<String, Account> updateAccounts = new Map<String, Account>();
        
        // To perform the logic on Account
        accounts = performLogic(accounts);
        
        // Condition to check for the size of the List of Accounts after performing the logic
        if(accounts.size() > 0) {
            // Loop to iterate over the list of Accounts that you want to update
            for(Account account : accounts) {
                Account updateAccount = new Account(Id = account.Id, Name = account.Name, Avg_Time__c = account.Avg_Time__c);
                updateAccounts.put(updateAccount.Name, updateAccount);
            }
        }
        
        // Preforming the DML operation
        if(updateAccounts.size() > 0) {
            update updateAccounts.Values();
        }
    }
    
    // Method to excuete on Before save of Account Records
    public static void onBeforeUpdate(List<Account> accounts){
        // To perform the logic on Account
        accounts = performLogic(accounts);
    }
    
    // Method to perform the logic on Account Records
    private static List<Account> performLogic(List<Account> accounts) { 
        Set<Decimal> accNumSet     = new Set<Decimal>();
        Set<String> accNameSet = new Set<String>();
        
        // Loop to iterate over the list of accounts 
        for(Account account : accounts) {
            accNumSet.add(account.Account_Number__c);
            accNameSet.add(account.Name);
        }
        
        // condition to check for the prepared sets
        if(accNumSet.size() > 0 && accNameSet.size() > 0) {
            Map<String, account> accountMap = getAccounts(accNumSet, accNameSet);
        }
        
        return accounts;
    }
    
    // Method to get the existing accounts from database
    private static Map<String, account> getAccounts(Set<Decimal> accNumSet, Set<String> accNameSet) {
        Map<String, account> accountMap = new Map<String, account>();
        
        List<Account> accountList = [SELECT ID, Account_Number__c , Name, Lead_Time__c, Avg_Time__c   
                                     FROM Account 
                                     WHERE Name = :accNameSet 
                                     AND Account_Number__c = :accNumSet];
        
        // Condtion to check for the List of accounts that returned based on the SOQL query
        if(accountList.size() > 0) {
            Decimal avgTime = 0.0;
            Integer size = 0;
            // Loop to iterate over the query results
            for(Account account : accountList) {
                
                // Condition to check if the account name is already exists in account Map
                if(accountMap.containsKey(account.Name)) {
                    size = size + 1;
                    avgTime = +account.Lead_Time__c /size;
                    account.Avg_Time__c = avgTime;
                    accountMap.put(account.Name, account);
                } else {
                    size = size + 1;
                    avgTime = +account.Lead_Time__c /size;
                    account.Avg_Time__c = avgTime;
                    accountMap.put(account.Name, account);
                }
            }
        }
        return accountMap;
    }
}

Test class with 100% code coverage:
----------------------------------------------------------
@isTest
public class AccountTriggerHandlerTest {
    
    @testSetUp static void prepareTestData() {
        
        Account account = new Account(Name = 'WXYZ', Account_Number__c = 1, Lead_Time__c = 2);
        insert account;
        
        List<Account> accounts = new List<Account>();
        for(Integer i=0; i<2; i++) {
            Account account1 = new Account(Name = 'XYZ', Account_Number__c = 2, Lead_Time__c = 2);
            accounts.add(account1);
        }
        insert accounts;
    }

    @isTest static void AccountInsertTest() {
        Account account = [Select Id, Name from Account Where Name = 'WXYZ'];
        System.assert(account != null);
    }
    
    @isTest static void AccountUpdateTest() {
        Account account = [Select Id, Name from Account Where Name = 'WXYZ'];
        update account;
    }
}

Hope this will help you.