RAID5/6+ is only as fast as your slowest disk, and seek times are no better. In most respects, mirroring gets you better performance, and for most drive configurations, is more reliable than RAID5. Massive throughput on single files one at a time is not interesting to most people with big storage requirements.
I currently run RAID6 (raidz2) across 10 drives. But I'll be moving to RAID10 before long. Whether using zfs or not, depends on btrfs stability.
RAID5/6 is only as fast as the slowes disk regarding ZFS yes if you talk about random IOPS. BTRFS performance I haven't measured yet.
RAID5/6 random IOPS can be quite good or as you can expect if you use MDADM.
The best solution seems to be tons of RAM for caching and ZFS with SSD SLOG if you need reliable and fast sync writes. But with ZFS, performance comes second over reliability.
I currently run RAID6 (raidz2) across 10 drives. But I'll be moving to RAID10 before long. Whether using zfs or not, depends on btrfs stability.