It is possible to have Fastrack AND a L2TP VPN setup without the VPN feeling 'sluggish':
First off we 'mark' the ipsec connections for identification:
/ip firewall mangle add action=mark-connection chain=forward comment="mark ipsec connections to exclude them from fasttrack" ipsec-policy=out,ipsec new-connection-mark=ipsec
/ip firewall mangle add action=mark-connection chain=forward comment="mark ipsec connections to exclude them from fasttrack" ipsec-policy=in,ipsec new-connection-mark=ipsec
Then we add a fasttrack rule to fasttrack all connections EXCEPT those marked by the mangle commands above:
/ip firewall filter add action=fasttrack-connection chain=forward comment="Fast Track everything except IPSEC" connection-mark=!ipsec connection-state=established,related
Move this rule to just below where the factory/original fasttrack rule is and disable/delete that original rule.