I use ProcessPoolExecutor in concurrent.futures which is a more user friendly wrapper to multiprocessing.pool for wimps like me.
Separately I am trying to answer a very reasonable question a friend asked me about the more limited diversification opportunities from futures relative to cash markets (there are of course a lot more of the latter). My answer was that it was either hard or expensive or both to trade certain assets classes through the cash markets. Examples of these are commodities, STIR and volatility. Do people agree with this answer? If I recall correctly Jerry in the podcast is a big fan of the cash markets (which presumably is why he also prefers to trade slowly) as a diversifier.
So I have a lot of experience with this, since one of my main achievements was adding a lot of non futures markets to the AHL portfolio.
As well as costs and logistical issues, there are problems leveraging and going short cash markets; plus the difficulty of modelling them compared to futures markets (for the latter to get to a total excess return series you just have to do rolls and backadjustment, fiddly but easy once you know how; and then contract selection and rolling when you're actually trading).
Stocks:
Mostly cheap to trade and no issues with small tick sizes. Vast market with plenty of diversification. Leverage is relatively difficult to find, you eithier use CFDs (UK) which are expensive, or a margin account in the US (on which the interest margins are also quite steep for retail). It's not always easy to go short. There are technical difficulties with systematic trading of stocks: companies dying, dividends, stock splits, scrip issuance, mergers and takeovers... Plus if you're going to use leverage and go short you need history or estimates of funding and stock borrowing cost.
ETFs:
Similar to stocks, although can be simpler especially if you avoid ETFs that pay dividends, and don't worry about borrowing or going short. History tends to be limited, but can be a useful way of filling in gaps in futures markets especially for corporate bonds and EM equity indices.
Commodity markets:
Cash markets really are only for the industry professionals. Transaction costs and logistical difficulties abound. I suppose at a push you could buy some physical gold and hold it as part of your overall portfolio, if you have a decent safe in your house... but trading it would cost a fortune.
STIR:
There isn't really a STIR cash market per se. The reference rate (although this is changing) is supposed to be based on interbank lending, so really you need to be a bank.
Volatility:
You're basically talking about option trading. Leverage comes for free. Spreads can be large for OTM options. There are technical difficulties in dealing with different strikes and expiries, building surfaces, translating between notional 25 delta strangles and what you can actually trade.
Bonds:
The corporate bond market is highly fragmented, and for retail traders the spreads to be wide. Government bonds are better, and I think in the US at least it's relatively easy to buy bonds as a retail trader, but not in the UK. Leverage isn't too bad, but again for retail traders not cheap. They are technically harder to fit into a systematic strategy due to coupons, ageing... but not as bad as stocks.
You're better off with ETFs.
OTC fixed income:
Swaps et al. Basically almost inaccesible to retail traders. But relatively easy to turn into synthetic futures.
Bitcoin:
Buying and trading cash bitcoin is still (IMHO) a fraught, difficult and dangerous business.
Summary:
The most attractive cash markets are ETFs, stock and vol. It's no coincidence that most CTAs that have expanded into other asset classes are focused mainly in these areas. A lot of time and investment would be needed to build backtesters and trading strategies for stock and vol trading, but you will be rewarded with uncorrelated strategies. For stocks you are probably better off focusing on long only, or perhaps long only with a futures hedge.
ETFs are probably an easier proposition, and they are a nice way of covering gaps in your futures exposure. In many ways they are better than the cash markets they represent. They are relatively easy to get total return series for, if you exclude dividend payers. Trading them on a long only basis is going to be quite nice and easy, although obviously not as capital efficient as futures trading.
GAT