If CA counted all its votes in one day, the argument would be that they weren’t doing it properly and were just eyeballing ballots and not double checking things. No matter what, they’d find an excuse to claim fraud because the process isn’t what concerns them, but the results.
Two things can be true:
1. Trump is alleging voter fraud in California without providing evidence.
2. California’s delay in counting votes is embarrassing and unacceptable. Even if you don’t think it’s fraud it shows pure incompetence. There’s really no way around it.