Very slow cranking

How about a quick question, was this the problem that you tore out the old parts to repair or did this happen after all the new parts were installed? Attached is the starting circuit. The red and black circled wires are the heavy battery cables. The red one connects to the starter and the black to a ground on the engine and the body. They should be very heavy cable. Then one wire that goes to the starter connects to the starter motor on the circled terminal. The other wire with the S goes to the wiring to the starter relay.
The first thing to test would be the battery, even a new one can be bad. I would take it to be tested just to be sure.
Next to see if it's the starter or engine you can jump the starter. To do that take a heavy jumper wire and connect it to battery positive, then touch it to the terminal on the starter where the S tagged ire bolts on. If it still cranks slow you either have a bad battery or a bad starter, or there is a bad connection. To check the wiring you can read up on testing it here:
https://www.2carpros.com/articles/how-to-check-wiring

The main battery cables should be much heavier wire than that. The thicker the wire the more current it carries and starters take a lot of power to turn an engine over. The OE cable was a 4 gauge part. You can get the cables as pre made parts in many parts stores.
Once you have the heavier ones in place see if it starts properly.

The ground should be split. You want the ground from the battery to the engine, not from the battery to the chassis. Then run a second large ground from the engine to the chassis and a ground from the chassis to the body. That ties the grounds all together. With the main ground on the chassis as you have it now there is no ground path to the engine. Change that and then try it. Normally on Fords the large red goes to the solenoid you have the small wire on, then the second heavy cable comes from that same post to the starter then a third feed to the fuse box comes off the same post. However if you could get a picture of the starter that would help as Ford uses a couple different ones and you might have the older version. Those are wired different.

No rush we'll be here. Getting them set up right can be a chore sometimes.

The grounds sound good. Positive side sounds okay as well. The 4 gauge on the terminal is there to feed other circuits unless there is a feed from the starter, that depends on the way the OE wired it. The attached is the factory method and you should have a large battery feed to the fuse box. If the rest of the electronics work and just the slow cranking you likely have the feed from the starter.

The main one is the ground to the engine, when the starter turns it takes a lot of amperage and without a good ground it can do damage as the power finds a new path. So say you have only a good ground to the frame of the truck, you turn the key and the electrons go from the battery, through the starter and to the engine block, now there is no good ground so they look for a path of lowest resistance. Usually that is going to be through places where metal meets metal like through bearings, small areas of the engine mounts, other wiring to the engine.
Then you want the chassis to have a good ground because there are items mounted to it that take power to operate as well, and those need grounds. Lastly you have the body ground, that handles things like the lights, radio, dash and more. Generally I will add a heavier ground from the engine to the body as well as the smaller one near the battery. That way the small ground act's similar to a fuse, if the main larger ground fails the smaller wire will heat up and char the insulation, telling you that the main ground has an issue. For that ground the easy way is to use a simple braided ground strap like the image. Those work better than cables if you add them at the engine because the engine motion can break regular cable due to work hardening. The braid allows more flex. Usually I'll put those between a stud or bolt on the rear of the engine over to a good connection on the firewall.
Then you know that everything attached to the truck has a good ground.