Seems that Encryption is required.
About speed. RC4 encryption that is used on MPPE isn't slow.
Here is benchmark on OrangePi Zero - 1GHz/4cores:
ARC4 : 48984 KiB/s, 0 cycles/byte
3DES : 4104 KiB/s, 0 cycles/byte
DES : 11035 KiB/s, 0 cycles/byte
AES-CBC-128 : 15950 KiB/s, 0 cycles/byte
AES-CBC-192 : 13867 KiB/s, 0 cycles/byte
AES-CBC-256 : 12321 KiB/s, 0 cycles/byte
AES-XTS-128 : 13683 KiB/s, 0 cycles/byte
AES-XTS-256 : 10831 KiB/s, 0 cycles/byte
AES-GCM-128 : 7889 KiB/s, 0 cycles/byte
AES-GCM-192 : 7337 KiB/s, 0 cycles/byte
AES-GCM-256 : 6898 KiB/s, 0 cycles/byte
AES-CCM-128 : 6938 KiB/s, 0 cycles/byte
AES-CCM-192 : 6120 KiB/s, 0 cycles/byte
AES-CCM-256 : 5536 KiB/s, 0 cycles/byte
ChaCha20-Poly1305 : 19739 KiB/s, 0 cycles/byte
CAMELLIA-CBC-128 : 13663 KiB/s, 0 cycles/byte
CAMELLIA-CBC-192 : 10609 KiB/s, 0 cycles/byte
CAMELLIA-CBC-256 : 10595 KiB/s, 0 cycles/byte
ChaCha20 : 28048 KiB/s, 0 cycles/byte
BLOWFISH-CBC-128 : 19464 KiB/s, 0 cycles/byte
BLOWFISH-CBC-192 : 19369 KiB/s, 0 cycles/byte
BLOWFISH-CBC-256 : 19465 KiB/s, 0 cycles/byte
With speed of almost 49MBps it's fastest than everything else.
On x86:
ARC4 : 614310 KiB/s, 5 cycles/byte
3DES : 31550 KiB/s, 108 cycles/byte
DES : 75700 KiB/s, 45 cycles/byte
AES-CBC-128 : 628398 KiB/s, 5 cycles/byte
AES-CBC-192 : 575218 KiB/s, 5 cycles/byte
AES-CBC-256 : 530374 KiB/s, 6 cycles/byte
AES-XTS-128 : 493825 KiB/s, 6 cycles/byte
AES-XTS-256 : 431153 KiB/s, 7 cycles/byte
AES-GCM-128 : 223130 KiB/s, 15 cycles/byte
AES-GCM-192 : 216002 KiB/s, 15 cycles/byte
AES-GCM-256 : 209600 KiB/s, 16 cycles/byte
AES-CCM-128 : 339657 KiB/s, 10 cycles/byte
AES-CCM-192 : 318655 KiB/s, 10 cycles/byte
AES-CCM-256 : 294662 KiB/s, 11 cycles/byte
ChaCha20-Poly1305 : 283703 KiB/s, 12 cycles/byte
CAMELLIA-CBC-128 : 99080 KiB/s, 34 cycles/byte
CAMELLIA-CBC-192 : 76184 KiB/s, 44 cycles/byte
CAMELLIA-CBC-256 : 76186 KiB/s, 44 cycles/byte
ChaCha20 : 398711 KiB/s, 8 cycles/byte
BLOWFISH-CBC-128 : 107072 KiB/s, 31 cycles/byte
BLOWFISH-CBC-192 : 107073 KiB/s, 31 cycles/byte
BLOWFISH-CBC-256 : 107073 KiB/s, 31 cycles/byte
It's also fastest (honestly 2nd place) - over 600MBps.
So MPPE isn't reason to slow down your PPTP VPN connection.
And i don't see reason why to want to disable it.