The errata to the DMG added a section “Combining Game Effects” which states:
[…] When two or more game features have the same name, only the effects of one of them — the most potent one — apply while the durations of the effects overlap […]
There are two similar effects, one is the Stone Golem’s Slow feature which states:
The golem targets one or more creatures it can see within 10 feet of it. Each target must make a DC 17 Wisdom saving throw against this magic. On a failed save, a target can’t use reactions, its speed is halved, and it can’t make more than one attack on its turn. In addition, the target can take either an action or a bonus action on its turn, not both. These effects last for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
And the other is the slow spell which states:
You alter time around up to six creatures of your choice in a 40-foot cube within range. Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.
An affected target’s speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn.
If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn’t take effect until the creature’s next turn, and the creature must use its action on that turn to complete the spell. If it can’t, the spell is wasted.
A creature affected by this spell makes another Wisdom saving throw at the end of each of its turns. On a successful save, the effect ends for it.
There is a question on “Can you dispel the Slow effect of a Stone Golem?” where the answer is that you cannot as the Golem’s feature is not actually a spell (the features are different features). That said, they do have the same name, so I am unsure how they interact. Many of the effects stacking would be redundant, but your speed could be halved twice and the two effects could have different save DC’s.
Do the Stone Golem’s Slow feature and the slow spell combine, or does the “Combining Game Effects” section prevent that because they technically share the same name?