That is a myth.
Standard for each or for each row are not slow activities at all. Please review the results of the test in this thread from @postwick There is no need to try to avoid For Each loops on large amounts of data
In your case, you have to educate your customer and walk them through why it is advantageous to choose standard UiPath activities. There are numerous advantages of using standard activities. To name a few, code maintenance, code review and easy of code upgrades in the future.
If it is a strict requirement (only use linq), the tech lead or the solution architect should have intervened and informed the customer that there are no performance advantages by only using linq query. This would have ensured you as developer would not be stuck with a problem which a simple for loop could have fixed (without affecting performance).