ElGamal and Schnorr groups
Note the obvious: for Elgamal signature generation you must share parameters between users. If I (re-)read Elgamal encryption then it seems to me that parameter $p$ or $q$ (the safe prime) need to be shared in advance as well. Even though the generation of safe primes may be time consuming, you don't need to do this for each signature generation or decryption - the parameters may be static.

As for the key size, the key size of the prime must be about as large as those for RSA and DH keys. This is because the security is related to the hardness of the Diffie-Hellman problem. This means that you may look up the sizes on sites like keylength.com and look for the sizes required for "Discrete Logarithm" or the sources, for instance of NIST (US) or ECRYPT II (Europe), it references.

So there is no need to use primes that have additional properties over those required for Elgamal signature generation or encryption. Any generation of large primes will take large amounts of time compared to encryption/decryption. If you want efficiency then you can share (named) parameters.