People will complain that that ride is short, but shorter coasters make for more throughput of people, so it's a tough balance.
Those 2 things really aren't correlated. A shorter ride doesn't necessarily mean higher throughput. Shorter just means shorter. You could have a 15 second long coaster than only has 1 car on the track which fits 2 people in it and takes 1 minute to load. That wouldn't have a very good hourly throughput despite the ride only being 15 seconds long. Or you could have a 5 minute long coaster which can accommodate 15 trains on the track at once which each hold 32 guests and can be dispatched in 30 seconds. Which one will have higher throughput?
The real key to throughput for a coaster are:
1. How many trains can be on the track at the same time?
i.e. how many blocks is the track split up into. Each track block can only have a single train in it. No other train can enter the same block of track until the block is fully cleared by the previous train. This is how collisions are prevented. Longer coasters often have multiple mid-course brake runs where a train can be stopped, which means the track is broken into more blocks, which allows more trains to be on the track at the same time. This is why longer coasters don't mean lower throughput. Looking at the track layout of Slinky Dog and that there is only one place mid-course to stop the train, I'm going to guess they will run 3 trains on this track simultaneously. Possibly 4 depending on the length of the final brake run (tough to tell from the overheads if it can fit 2 trains there).
2. How many guests fit on each train?
If the animation released by Disney is accurate, it looks like there will be 10 rows on each train, with 2 guests per row. That's only 20 guests per train. So not very many as far as coasters go.
3. How quickly can trains be unloaded, loaded, and dispatched and how long will the load station be empty between trains?
This part will be crucial. From the overheads it looks like there will possibly be a separate unload zone and load zone as on 7DMT, Everest, and Rock 'n Roller Coaster. This will help keep the trains moving, so that's a good sign if I'm seeing it correctly. Until the ride is in operation we won't know how efficient they will be. With only 10 rows to check and Disney's penchant for cutting labor costs, how many cast members will be on restraint check duty? If only 2 that may cause them to not be incredibly efficient with the dispatches.
So there are many factors in guest throughput, but the ride itself being shorter isn't really one of them. Example:
Expedition Everest is a much longer coaster than Slinky Dog, but I guarantee its hourly guest throughput is way higher than Slinky Dog will be. Everest fits 32 guests per train, has separate unload and load stations, has enough track blocks to operate 5 trains at a time, and has an efficient dispatch process. That's why it has a nice high hourly throughput of probably 2000 guests per hour.
The Barnstormer is a much shorter coaster than Everest but it's hourly throughput is likely only 25% that of Everest's. It has trains that only fit 16 guests, it has a shared unload/load zone, it can only have 2 trains on the track, and has less cast members checking restraints. That's why it has a very low hourly throughput of maybe 500 guests per hour (likely lower).