LLC is more overhead, so depending on the packet size you are transmitting / receiving this could mean one more ATM cell so for a ~ 1500 byte packet that would mean 3% speed loss. But if the MTU or packet size you happen to have means that the overhead will fit into one ATM cell with or without the extra overhead of LLU compared with VC-MUX then there is no speed difference in that one particular case, but there will always be _some_ cases where LLC is worse, depending in the length.
If you use PPPoEoA IP MTU 1500 (1500 + 8 = 1508 baby jumbo frames on PPPoEoE, on a router-to-modem link) the LLC will cost you that extra 3% as it pushes you into another almost completely wasteful ATM cell. If you use the common IP MTU 1492, then using LLC won't make any difference on full length packets.