The 50% mount is a bit slower but I didn't have too much trouble keeping up with the 60% mounts. The only time I really remember lagging behind and getting picked off was trying to cross Chaos Wastes to get to the Fort. Luckily, I was in a guild warband and they came back for me. But overall, the 50% mount can generally keep up and hopefully any good WB leader with lower level players will be aware of the tail and pause periodically to let people catchup. That's another huge benefit of being in a guild and/or being in voice chat so you can call things like that out.
By way of comparison, I just recently started playing Elder Scrolls Online where you can upgrade your mount speed 1% per day up to 60%. I've only been playing a few weeks so my mount is only +15% compared to everyone else fully upgraded mounts with +60% max speed plus max stamina. The speed difference is massive and I still have 45 more days of upgrades to go before I can even be equal.