Database is slow after server transfer
The same Procs takes 2 to 5 times more to run sometimes (it seems to vary).


You should recompile the stored proc using sp_recompile 'procedureName'. This should be one of the post migration step.

Another thing to be aware is if you are going to use more max memory (since you have more RAM on new server), you should look into testing and enabling TF2335

Also check the power option on the server. It should be set to high performance.

I agree with @Phil, check with your VM admin to see if the host machine is over committed in-terms of resources or not. Depending on if you are using VMWare  there are things like balloon driver (pdf whitepaper). For Hyper-V, read Running SQL Server with Hyper-V Dynamic Memory whitepaper.