From my experience:

When working with Magento 2 (especially on your local machine), you need to enable Cache. Using command line to clean or flush cache if you want to see your new change.

For the static contents(js, html templates) under pub/static, you can edit directly the file to see the new changes. Need to disable or clear Browser Cache also. If still not see the changes, try to clear Magento Cache. If it works, copy it to the "real" file in your module.

You should set the mode to developer mode during development, because, you will don't need to run di compile again and again.

It seems my main problem is session concurrency happening for whatever reason. I tried to use session caching with Redis as proposed in the documentation.

'session' =>
array (
  'save' => 'redis',
  'redis' =>
  array (
    'host' => '127.0.0.1',
    'port' => '6379',
    'password' => '',
    'timeout' => '2.5',
    'persistent_identifier' => '',
    'database' => '2',
    'compression_threshold' => '2048',
    'compression_library' => 'gzip',
    'log_level' => '1',
    'max_concurrency' => '6',
    'break_after_frontend' => '5',
    'break_after_adminhtml' => '30',
    'first_lifetime' => '600',
    'bot_first_lifetime' => '60',
    'bot_lifetime' => '7200',
    'disable_locking' => '0',
    'min_lifetime' => '60',
    'max_lifetime' => '2592000'
  )
),

But as I have the session locking problem I think this comes into account:

break_after_frontend

Number of seconds to wait before trying to break the lock for frontend (that is, storefront) session.

Meaning there is always a delay of max. 5 seconds. When I switched to memcached session it got slightly better because the wait was lower. It defaults to these settings:

memcached.sess_lock_wait_min => 1000
memcached.sess_lock_wait_max => 2000
memcached.sess_lock_retries => 5

These are the defaults from some older php-memcached version. The newest version proposes these defaults:

memcached.sess_lock_wait_min => 150
memcached.sess_lock_wait_max => 150
memcached.sess_lock_retries => 200

After reading these two: https://github.com/php-memcached-dev/php-memcached/pull/350 https://github.com/php-memcached-dev/php-memcached/issues/269

I tried with:

ini_set('memcached.sess_lock_wait_min', 150);
ini_set('memcached.sess_lock_wait_max', 150);
ini_set('memcached.sess_lock_retries', ini_get('max_execution_time') * 1000 / 150);

in Magento's index.php and this again speeds up the process. I now get a load of the checkout in roughly 4 seconds. The cart is a bit slower with ~5 secs because of the loading of totals-information. (all first try no browser caching)

I suppose fixing the session locking problem would again speed up the loading times as looking at the profiles reveals there is still a wait for session_start() from 300 to 800 ms for several calls.