|Level||Workload Slot||Cost for next upgrade||Note|
|1||4||$1 Trillion & 16 Oil & 10 Building Materials|
|2||8||$4 Trillion & 200 Oil & 20 Building Materials|
|3||12||$50 Trillion & 2,000 Oil & 40 Building Materials|
|4||20||$5 Trillion & 4,500 Oil & 80 Building Materials|
|5||25||$6 quadrillion & 6,000 Oil & 160 Building Materials|
|6||30||$100 quadrillion & 8,000 Oil & 320 Building Materials|
|7||35||$1 Quintillion & 11,000 Oil & 500 Building Materials|
|8||40||$50 Quintillion & 15,000 Oil & 750 Building Materials|
|9||45||$1 Sextillion & 20,000 Oil & 1000 Building Materials|
Each catalyst, when applied to a current craft, will reduce the remaining time for the craft by 20%, at the cost of using one more slot.
If you are trying to determine the most efficient use of time, you will want to calculate how many "slot-minutes" are used by a recipe. Building a green gem with no catalysts runs for 240 minutes and uses 2 slots, for a total cost of 480 slot-minutes. Building a green gem with 10 catalysts runs for 25.75 minutes and takes 12 slots, for a total cost of 309 slot-minutes. Building the catalysts takes 5 slot minutes each, or a total of 50 slot minutes for 10 catalysts. The overall cost is 359 slot minutes.
Since the catalysts reduce the amount of time by a factor of 4/5, it is generally wise to use them as long as the number of slots you are using increases by less than 5/4.
Put another way, lets say you are making some gem and are currently using 4 slots. if you add another catalyst, it will increase the number of slots by a factor of 5/4 and decrease the remaining time by a factor of 4/5. Since these two variables are multiplied together for the total time cost, the total cost will break even, no matter which gem you are making, and no matter how much time is left for the gem.
If the number of slots you are currently using is 5 or more, it is always worth it to add a catalyst (negating the costs of making the catalyst, of course).
Since yellow gems require 6 slots to craft, it is always worthwhile to make just one at a time, and use your remaining slots for catalysts when making them.
Since purple gems require 3 slots to craft, you are costing yourself efficiency if you use 3 slots or less for catalysts, but using 4+ slots for catalysts is definitely worth it. My advice is that as long as you have 8 or more slots available, you should make one purple gem at a time and use the remaining slots for catalysts.
Green gems use 2 slots, and adding catalysts will hurt the efficiency unless you can add at least 8 catalysts. If you have 12 slots available, then using them all for catalysts on making a single green gem will take a total of 359 slot minutes, and that includes the 50 slot minutes required to build the 10 catalysts used. 359 is considerably less than the original 480, so if the other costs (oil, Pu, and U) are not important, then it may be worth it. If you have 20 slots, then you can reduce the total cost to 176 slot minutes. Of course, making green gems one at a time requires more babysitting, so I think it is up to each person to decide if it is worth it or not.
Red and Blue gems should never be built with catalysts. Once you start adding 5 minutes per catalyst for the cost of making the catalysts, the overall efficiency goes down with every catalyst you add.